The invention relates to a load detecting method and means for automatic storage and retrieval apparatus in which an automatically operated load-bearing stacker is movable down an aisle between a pair of spaced storage racks containing a plurality of vertically and horizontally spaced bins, or bays, the open ends of which confront one another on opposite sides of the aisle. The stacker conventionally has a vertically movable carrier which supports a retractable fork mechanism on a trolley that is movable laterally out of either side of the lifting carriage to service the racks. A combination receiving and discharge station may be located at the head of the aisle where incoming loads are picked up by the fork mechanism for deposit in a preselected bin and where outgoing loads are deposited by the fork mechanism after removal from one of the bins in response to instruction signals from a programmable controller.
Mechanical feelers were provided on earlier prior art stackers which, when the carrier had been positioned in registry with a preselected bin, projected into the bin to detect the presence or absence of a load therein. Such mechanical feelers had the disadvantages that they might not detect the presence of a load positioned near the back of a bin and also that the mechanically operated parts from which they were constructed often accidentally became damaged during operation or, in time, wore out and had to be replaced. More recent stackers have load detectors which utilize reflected light rays and photocells for sensing the presence or absence of a load in a preselected bin of a storage rack. However, such stackers utilizing reflected light rays to detect a load necessitate installation and maintenance of brackets supporting reflectors at the rear of each bin which involves considerable initial and maintenance costs and does not provide a fail-safe indication of the presence or absence of a load in the bin.